docs: def calculate_z1pt0(vs30):
      '''
      Reads an array of vs30 values (in m/s) and
      returns the depth to the 1.0 km/s velocity horizon (in m)
      Ref: Chiou & Youngs (2014) California model
      :param vs30: the shear wave velocity (in m/s) at a depth of 30m
      '''
      c1 = 571 ** 4.
      c2 = 1360.0 ** 4.
      return numpy.exp((-7.15 / 4.0) * numpy.log((vs30 ** 4. + c1) / (c2 + c1)))